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Difficulty: MediumSoldering Equipment

During a quality inspection of a circuit board assembly, a technician observes a solder joint with a dull, grainy surface and poor mechanical adhesion. What is the most likely cause of this cold solder joint defect?

  1. A
    Applying rosin-core solder directly to the heated workpiece rather than melting it on the iron tip
  2. Movement of the component lead before the molten solder had completely solidifiedAnswer
  3. C
    Connecting the soldering station chassis to an earth ground to dissipate static charges
  4. D
    Wiping the soldering iron tip on a damp cellulose sponge between joints

Answer

Movement of the component lead before the molten solder had completely solidified
A cold solder joint commonly occurs when a wire or component lead moves while the molten solder is solidifying, or when the joint was not heated sufficiently. This mechanical disturbance fractures the crystalline structure of the alloy, leaving a dull, rough surface with high electrical resistance and low mechanical strength.

Step-by-Step Solution

1
Analyze the physical symptoms described in the stem (dull, grainy appearance and weak mechanical bond).
Identify the condition as a classic cold solder joint defect.
A sound electrical solder joint has a shiny, smooth, concave fillet; roughness and lack of shine indicate improper cooling or insufficient thermal transfer.
2
Evaluate the workshop procedural causes that produce crystalline disruption in molten solder.
Physical disturbance or premature heat removal during the alloy's transition phase fractures the joint structure.
If parts move while the molten alloy cools, the metallic bonds are sheared, resulting in a high-resistance, brittle connection.

Key Concept

Soldering joint defect diagnosis and thermal cooling integrity
Estimated Time:45s
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Topics

Soldering Equipment

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